I do believe that this number is rounded by car and driver. It may have been so close to 4,000 with a driver that they just stated it as is. Based on some of the older posts here and over at AZ of people posting some pics of the weight on the S4, Audi accurately states it between like 3800-4000.

All of the above weight (curb and maximally optioned) is for the B8 S4, S5 and RS 5 is with USA specifications.

The supercharged gasoline 3L V6 engine and Borg Warner seven-speed S-Tronic direct shift gearbox (or DSG for short) transmission (both of which are described in numerous full details in the fourth paragraph of this reply message above) are already in the B8 S4, S5 convertible and S5 sport-back worldwide. With this engine, the DSG transmission (which is the DL550 for longitudinally mounted engines with a maximum capacity of 406 lbs-ft of torque/550 nm, hence the DL550 designation) is the only optional transmission worldwide on the B8 S4, S5 sport-back and, eventually, the S5 coupe. In North America, this DSG transmission is mandatory for the S5 convertible.

The RS 5 will have a regularly aspirated gasoline DOHC 32V 4.2L FSI V8 with (or without) valve-lift (with an 8,600 RPM redline) producing 444/450/331 SAE hp/DIN hp/kw @ 8,400 RPM. The torque produced by this engine in the RS 5 will either be 332 lbs-ft of torque/450 nm from 4,000 to 6,000 RPM or 339 lbs-ft of torque/460 nm from 4,500 to 6,500 RPM.

The RS 5 may only come with the yet to be released Borg Warner DL750 direct shift gearbox (or DSG for short) for longitudinally mounted engines with a maximum capacity of 553 lbs-ft of torque/750 nm, hence the DL750 designation) worldwide. Or this DSG transmission will be the only transmission in North America (optional overseas) with the Getrag ML600 (manual transmission for longitudinally mounted engines with a maximum capacity of 443 lbs-ft of torque/600 nm, hence the ML600 designation). Or, worldwide, this manual transmission will be standard with this DSG transmission optional for the RS 5 worldwide (including North America, of course ).